1. Field of the Invention
This invention relates to the gas phase fluid bed polymerization of olefins. More particularly, this invention relates to minimizing the venting or flaring of unreacted monomer. This invention especially relates to recovery of unreacted monomer from effluent gases and employing such recovered gases to purge unreacted monomer from the polymer product.
2. Description of the Prior Art
Olefin polymerization has been practiced commercially for a number of years. Polymers have been produced successfully by both continuous and batch processes employing such techniques as solution polymerization, slurry polymerization and polymerization in loop reactors.
For several years considerable interest has been shown in the commercial development of gas phase olefin polymerization. One particularly useful technique has been employing a fluidized bed reactor for the catalytic polymerization of gaseous monomers to produce solid particulate polymers. Fluidized bed reactors for the production of particulate polymers can be operated by continuously feeding a dry, powdery catalyst and a gaseous stream containing the polymerizable monomer or monomers to a fluidized bed composed of the particulate polymer product. The polymer is kept in a fluidized state in a rising stream of the gaseous feed mixture containing the olefin or olefins to be polymerized. The unreacted monomers and any carrier gas which may be employed are passed out of the reaction vessel and recycled, at least in part, and returned to the bottom of the reactor where additional gaseous feed is introduced. The combined mixtures are passed through the fluidized bed of polymer to maintain it in a fluidized state and continue the polymerization reaction. The exothermic heat of reaction from the polymerization is conveniently removed by passing the recycling gaseous mixture through appropriate heat exchange equipment before it is returned to the lower portion of the reactor. The polymer product produced may be conveniently removed from the fluidized bed reactor by a series of lockhopper and valve arrangements.
The patent art contains numerous patents related to fluid bed olefin polymerization, especially the polymerization of alpha-olefins, such as ethylene alone or in combination with other alpha-olefins, particularly C.sub.3 -C.sub.8 alpha-olefins, see, e.g., U.S. Pat. Nos. 3,922,322 of Roger, et al.; 4,003,712 of Miller and 4,011,382 of Levine, et al.
A quantity of unreacted monomers usually remains in or on the particle of olefin polymer product removed from the reactor. These unreacted monomers should be removed from the polymer product, since they can present a danger of explosion in contained spaces such as product storage silos or tank cars if the unreacted monomer concentration reaches dangerous levels in the presence of oxygen. Further, environmental standards concerning hydrocarbon emissions require effective and proper disposal of any unreacted monomer removed from the polymer product.
The prior art teaches a number of techniques for removing unpolymerized monomers from the polymers prepared from the corresponding monomers. For example, U.S. Pat. No. 4,372,758 of Bobst, et al. discloses a degassing or purging process employing an inert gas, such as nitrogen, for removal of unpolymerized gaseous monomer from solid olefin polymers, particularly those produced in a gaseous phase polymerization. Solid polymer is conveyed to the top of a purge vessel by means of an inert gas stream, an inert gas stream is introduced into the bottom of the purge vessel in such a fashion that the solid polymer is counter-currently contacted with the inert gas purge stream to strip away the gaseous monomers from the solid polymer, and the resulting stream of inert gas and unreacted gaseous monomer is recovered from the purge vessel. The resulting stream or a portion thereof may serve as the conveying gas or, in another embodiment, as the purge stream.
In addition, since the polymer is produced in a fluidized bed operating at above atmospheric pressure, the removal of the polymer from the reactor to a zone of atmospheric pressure necessarily removes a significant quantity of the unreacted monomer merely because of the change in pressure.
Regardless of the technique involved, the unreacted monomer is often admixed with an inert gas stream which, depending upon circumstances, is often passed to a flare for disposal or vented to the atmosphere. Regardless of the technique employed, the unreacted monomer represents a significant loss of monomer which in other instances could be recycled for reaction within the fluidized bed, and the concomitant flaring or venting of the inert gas represents a not insignificant operating cost, as well as a desirably avoided environmental factor.
It is an object of this invention to provide a process for the removal of unreacted monomer from a polymer product by a technique which permits the recycling of the unreacted monomer to the reaction vessel.
It is another object of this invention to minimize or eliminate the flaring or venting of unreacted monomer from a fluid bed olefin polymerization process.
It is a further object of this invention to minimize or eliminate the quantity of inert gas employed for transporting polymer product within the polymerization plant and for purging unreacted monomer from the polymer product.
The achievement of these and other objects will be apparent from the following description of the subject invention.